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We were told last night that Ed Miliband would be talking about an "explosion" in zero-hours contracts, although in the event he has changed it to an "epidemic".

Why might he have done that?

One of the more remarkable things about zero-hours contracts is how little we know about them, starting with what they are.

The Office for National Statistics (ONS) refers to them as contracts that do not guarantee a minimum number of hours. It has , neither of which is entirely satisfactory.

The first is based on asking respondents to its Labour Force Survey (the one from which we get the unemployment figures) whether they are employed in their main job on a zero-hours contract.

Between October and December 2014, around 697,000 people said they were, which was up from 586,000 in the same period the previous year. It's about 2.3% of all people in employment.

But there are caveats with this number, because it relies on people knowing whether they are employed under a zero-hours contract.

Remember that 45% of the increase was from people who had worked with such contracts for more than a year. In other words, a year ago they were also on zero-hours contracts but didn't say so.

So there might have been an explosion, or there might be growing awareness.

In order to get round the problem of people not knowing what sort of contract they have, the ONS produced a second figure by asking employers whether they employed people using zero-hours contracts.

It turned out that in the first two weeks of August 2014 employers had about 1.8m zero-hours contracts under which any work was done. But remember that's contracts, not people. One worker could have several such contracts.

The only previous figure for this measure was for January 2014, when there were 1.4m contracts. So the figure may have exploded, or it may be that seasonal industries such as agriculture and hospitality use more zero-hours contracts in August than they do in January.

Watchdog the UK Statistics Authority has been with complaints about the use of zero-hours statistics.

The latest figures also tell us that, on average, someone on a zero-hours contract works 25 hours a week. People on zero-hours contracts are more likely to want to work more hours - 34% said this, compared with 13% of people not on a zero-hours contract.

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